Controlling the speed or torque of an electromotor with a frequency converter permits a substantially lossfree adjustment of the motor to the requirements of the working unit driven by said motor. This applies especially to an arrangement with pumps and ventilators as working units, where the requirements of the arrangement change with respect to the variable delivery.
Externally installed static frequency converters are box-like devices emitting their dissipation heat to the ambient usually by free convection at a power range of up to approx. 100 kW. Owing to insufficient heat transmission the surfaces of such a device have to be large. Consequently the device itself becomes rather large, due to the fact that frequency converter requires an average volume of between 0.005 and 0.01 m.sup.3 /kW at a power range of up to 100 kW.
It is thus necessary to install a frequency converter separately. Owing to the comparatively long cables between the frequency converter and the electromotor this results in interferences caused by electromagnetic fields. In the future this will cause even heavier shielding because of government requirements. Furthermore, the mobility of the motor is curtailed by the large construction of the frequency converter. Finally the costs of such a frequency converter operating in low-power ranges will exceed those of the motor.